1. Field
The present disclosure relate to a trimming circuit to adjust fluctuation in circuit characteristics, a power supply including the trimming circuit, and a trimming method executed by the trimming circuit.
2. Description of the Background Art
In semiconductor integrated circuits, due to fluctuation in circuit characteristics, there is a difference between target values (e.g., upper limits of current and voltage), desired to be output from the semiconductor integrated circuit, and an actual output value (e.g., upper limits of detected current and voltage). Accordingly, the semiconductor integrated circuit includes a trimming circuit that adjusts the circuit characteristics (perform trimming) so that an output value is consistent with the target value. The trimming circuit, which is generally formed by a ladder resistor circuit, adjusts an output voltage of the ladder resistor circuit to perform trimming. In order to adjust the output voltage of the ladder resistor circuit, an adder to add a certain offset to the output voltage of the ladder resistor circuit and a multiplexer to correct rate of change (gradient) of the output voltage relative to the target value are used.
For example, in JP-2002-231887-A discloses a programmable trimming circuit to trim a reference voltage of an integrated circuit after the integrated circuit is mounted on a chip and is packed. In this example, the trimming circuit includes a first resistor, multiple programmable trimming cell circuits, a digital analog converter (ladder resistor circuit), and a second resistor. The first resistor is controlled to generate a sequence of a test bit signal and a sequence of a setting bit signal. The multiple trimming cell circuits, selectively connected to the first resistor, receive the test bit signal and the setting bit signal and generate output signals similarly to the received signals. The digital analog converter, constituted by the ladder resistor circuit, converts the output signals into trimming currents proportional to the output signals. The second resistor converts the trimming currents into trimming voltages and superimposes the trimming voltages on initial values of reference voltages generated by the integrated circuit.
In this example, when the reference voltage of the integrated circuit is trimmed, the initial value of the reference voltage generated by the integrated circuit is measured, the initial value of the reference voltage is compared with an accurately detected reference voltage, and a control signal indicating the difference therebetween is generated. Then, the multiple trimming cell circuits generate the bit sequence representing signs and magnitudes of the trimming currents, generate the trimming currents having the magnitude and sign proportional to the bit sequences, superimposing the trimming voltage converted from the trimming current, and determines whether or not the superimposed voltage is equal to the accurately detected reference voltage. When the superimposed voltage is suitable and equal to the accurately detected reference voltage, the trimming cell circuits permanently set the bit values when the bit sequence is generated.
In the above-described example, multiple trimming cell circuits respectively include fuses, and melt and cut the fuses to set the bit value “1” permanently. Accordingly, in this example, once trimming is performed, adjusting cannot be performed again.
Although the conventional trimming method is effective when linearity is present in the difference between the target value and the output value that is, when the output value (dependent variable y) changes at a constant rate (linearly) with the target values (independent variable x), otherwise, trimming cannot be performed. For example, when the semiconductor integrated circuit installing the trimming circuit is a power supply (e.g., switching regulator), since the characteristics fluctuate by generating current noise and heating as the output current of the power supply is increased, the upper limit of the output current (limit current) does not linearity change with the target values. Accordingly, when the power supply is operated in multiple modes having different upper limit currents respectively, performing trimming is difficult using the conventional method.
As detail example of semiconductor integrated circuit that is operated in the multiple modes having different upper limit currents respectively, a power supply of a universal serial bus (USB) that receives power from a USB host device via a USB interface.
At present, the USB is a standard to serial interface and is also used as an interface for supplying power.
The standard of “Battery charging specification revision 1.2” is developed considering charging to rechargeable batteries. “Battery charging specification revision 1.2” mainly defines ports of USB device having three types of power supplying capabilities including Standard Downstream Port (SDP), Charging Downstream Port (CDP), and Dedicated Charging Port (DCP). SDP is a port of host and hub conformed to USB 2.0. When the USB device is a low power device, the SDP has a power supply ability to generate a current of 100 mA, and when the USB device is a high power device, the SDP has a power supply ability to generate a current of 500 mA. CDP is a port of host and hub containing various power supply abilities and detection protocols conformed to USB 2.0. The CDP includes various types of ports having any one of power supplying abilities to generate the current ranging from 1.5 A to 0.5 A. DCP does not have a function as a downstream port but is a device to supply power via the port. The DCP includes various types of ports having any one of power supplying abilities to generate the current ranging from 500 mA to 5.0 A. Using a super speed port defined by USB 3.0, power of 900 mA can be supplied.
When the power is supplied from the USB host device by connecting the USB device to the USB host device (e.g., the power is supplied to charge the rechargeable battery of the USB device), the USB host device may include all of the power supplying capability. Accordingly, it is desired for the power supply of the USB device that receives the power supply to change the upper limits of the output currents of the power supply in accordance with the power supply abilities of the USB host device. However, in a configuration in which the power supply is operated in the multiple modes having different upper limit currents, performing trimming of the power supply with the conventional method is difficult.